How to return a PDF file from in-memory buffer using FastAPI?

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I want to get a PDF file from s3 and then return it to the frontend from FastAPI backend.

This is my code:

@router.post("/pdf_document")
def get_pdf(document : PDFRequest) :
    s3 = boto3.client('s3')
    file=document.name
    f=io.BytesIO()
    s3.download_fileobj('adm2yearsdatapdf', file,f)
    return StreamingResponse(f, media_type="application/pdf")

This API returns 200 status code, but it does not return the PDF file as a response.

fastapi boto3 amazon-s3 python
1 Answers

As the entire file data are already loaded into memory, you shouldn't be using StreamingResponse. You should instead use Response, by passing the file bytes (use BytesIO.getvalue() to get the bytes containing the entire contents of the buffer), defining the media_type, as well as setting the Content-Disposition header, so that the PDF file can either be viewed in the browser or downloaded to the user's device. For more details have a look at this, as well as this and this answer. Additionally, as the buffer is discarded when the close()method is called, you could also use FastAPI/Starlette's BackgroundTasks to close the buffer after returning the response, in order to release the memory. Alternatively, you could get the bytes using pdf_bytes = buffer.getvalue(), then close the buffer using buffer.close() and finally, return Response(pdf_bytes, headers=.... Example:

from fastapi import Response, BackgroundTasks

@app.get("/pdf")
def get_pdf(background_tasks: BackgroundTasks):
    buffer = io.BytesIO()
    # ...
    background_tasks.add_task(buffer.close)
    headers = {'Content-Disposition': 'inline; filename="out.pdf"'}
    return Response(buffer.getvalue(), headers=headers, media_type='application/pdf')

To have the PDF file downloaded rather than viewed in the borwser, use:

headers = {'Content-Disposition': 'attachment; filename="out.pdf"'}
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